1. Field of the Invention
The present invention relates to method and apparatus for processing an image suitable to an electronic file, a facsimile, a reader and a digital copier.
2. Description of the Prior Art
In a prior art image processing apparatus such as a copier or a facsimile which reads an original image by means of a solid-state imager such as a CCD and reproduces an image by means of a binary printer as represented by a laser beam printer (LBP), a so-called slice binarizing system is used in which pixel densities derived by reading the original image are compared with a constant threshold (which is usually an intermediate level between a white level and a black level) to binarize them when the original image includes a binary image text as represented by white and black images without gray level such as characters or a document, and a dither method is used when the original image includes gray levels or half tones as in, e.g. a photograph.
Usually, the original text is not classified to whether it is character text or photograph text but, in many cases, both texts coexist. Accordingly, an image zone separation processing system in which the type of image of the image zone is determined by a predetermined algorithm while the original text is read so that the binarizing system is selected, has been proposed and put into practice
FIGS. 1-4 show image zone-density distribution charts used by the proposed image zone separation processing system, in which an abscissa represents a one-dimensional image zone A and an ordinate represents a density D. A solid line represents a density distribution in the image zone of the original text, a broken line represents a reproduced density distribution, B represents a threshold for slice binarization and C represent a constant value (discrimination criterion) for discriminating an image content in the image zone. In FIGS. 1 and 2, the density distributions (solid lines) of the original texts are white to black and black to white, respectively. An image zone A(0) is determined as a binary image zone because of an abrupt density change, and the slice binarization is effected in this zone. Image zones A(-1) and A(1) adjacent to the binary image zone A(0) are determined to be gray level zones because a difference between a maximum density Dmax and a minimum density Dmin in each image zone is smaller than the predetermined value C, and the images are processed to the gray levels by the dither method. In the image zone A(0), the density variation .DELTA.D (=Dmax-Dmin) is larger than the predetermined value C (.DELTA.D&gt;C) and the reproduced image is black in a zone A"(0) in which the density D is larger than the threshold B, and white in a zone A'(0) in which the density D is smaller than the threshold B. Accordingly, the reproduced image in the image zone A(0) has a low fidelity (noisy) with only the white to black or black to white edge being emphasized.
In FIGS. 3 and 4, the image zone A(-1) is determined as a binary image zone because of the abrupt density change and slice binarization is effected in this zone. On the other hand, the zones A(0) and A(1) are determined as gray level image zones and the images are processed by the dither method. Accordingly, a noisy image with the black to white or white to black edge being emphasized is reproduced in the image zone A(-1) and the reproduced images are not exact reproductions of the original images.
FIG. 5 shows a block diagram of an image processing apparatus having the image discrimination function as described above. Numerals 3-1 and 3-2 respectively denote a maximum density measuring circuit and a minimum density measuring circuit which determine a maximum density Dmax and a minimum density Dmin of each of 4.times.4-pixel blocks of image or video data VD. Each of them comprises a RAM (not shown) having a capacity of (number of main scan pixels/4 pixels).times.(4 or 6 bits) and a comparator (not shown). The video data VD is supplied from a reader (not shown). Numeral 3-3 denotes a subtractor which calculates Dmax-Dmin for each block and numeral 3-4 denotes a comparator which compares the difference Dmax-Dmin with the predetermined value C and produces a one-bit image zone discrimination result R which is "1" for the binary image zone and "0" for the gray level image zone. The zone discrimination is effected at the fourth line scan. Numeral 3-5 denotes a RAM which latches the image zone discrimination result R discriminated at the fourth line scan until the end of the next fourth line scan. Numeral 3-6 denotes a slice binarizing circuit for binarizing the image line by line by a fixed threshold to produce a binary data Ds, and numeral 3-7 denotes a dither processing circuit for dithering the image line by line to produce a binary data Dd. The slice binarizing circuit 3-6 and the dither processing circuit 3-7 operate in synchronism with the above measuring circuits.
Since the image zone is discriminated at the end of the scan of four lines, it is necessary to delay the binary data Ds and Dd by four lines in order to select the binary data based on the discrimination result. Numerals 3-8 and 3-9 denote RAM's for that purpose. Since the discrimination result is required during the succeeding four line periods, the discrimination result is latched in the RAM 3-5 for four line periods.
S1 denotes a switch for selecting one of the binary data Dd or Ds in one block (image zone). It is switched by a gate circuit 3-10 depending on the discrimination result R.
The input video data VD is dithered or binarized, delayed by four lines and the binary data Dd or Ds is selectively outputted depending on the image zone discrimination result R. Thus, the binary image such as characters and the gray level image such as photographs which coexist on one original document is processed by the slice binarization processing or the dither processing. The binary data is supplied to a device such as a laser beam printer.
The image zone separation processing method in which the original image is divided into 4.times.4-pixel blocks, calculating the difference .DELTA.D between the maximum density Dmax and the minimum denisty Dmin in each block, and discriminating the block as the binary image zone if .DELTA.D is larger than the predetermined value C and as the gray level image zone if .DELTA.D is smaller, requires a relatively small circuit scale and is a practical method. However, when an original image having an abrupt change from a gray level which is relatively close to the white level to a gray level which is relatively close to the black level is to be reproduced, an image zone which includes a boundary of the change is discriminated as a binary image zone and the subsequent zones are treated as gray level image zones. As a result, the densities of the reproduced image distribute in the gray level close to the white level, white, black and the gray level close to the black level, and the abrupt white-to-black changes appear in the boundaries. Accordingly, a very noisy and inexact image is reproduced.
When the original image includes a character written by a thick black line on a white background, edges of the thick line are sharp because of the slice binarization processing but the inside of the thick line is dither-processed. As a result, the character is not exactly reproduced. When the processed image information is transferred, a transfer efficiency is very low.